Traditional Soil Survey

Traditional soil surveying is based on a classic concept known as the
soil-landscape relationship. Through extensive field investigations a
soil scientist first builds a conceptual model associating distinct soils
with particular landscape positions and other environmental clues. The
conceptual model is used with air photo interpretation to identify and
delineate soil-landscape units, forming soil polygons. This traditional
approach has several unavoidable limitations for efficient production
of soil spatial information. These limitations are related to the use
of a polygon-based model, the manual mapping process, and the lack of
documentation on the soil-landscape model.

Limitations

Polygon-based model

With the polygon-based model only soil bodies above a certain size (scale-dependent)
can be shown on the resulting soil map. Consequently, the level of detail
is limited by the scale of the map, not by what the soil scientist knows.
Also, the soils in a given soil polygon are often treated as homogenous
bodies; changes in the soil property values occur only at the boundaries
of the polygons. This creates a very unrealistic representation of spatial
variation of soil properties.

Manual mapping process

The second limitation is the manual mapping process, during which surveyors
manually delineate the extent of soil bodies based on visual interpretation
of environmental conditions. It is very difficult for soil mappers to
identify soil-landscape units using more than three environmental data
layers, due to the limits of human capacity for simultaneous visual perception
of multiple variables. As a result, the delineation of soil-landscape
units may not reflect the totality of knowledge possessed by the surveyor.
In fact, most soil mappers base their soil-unit delineation solely on
the visual interpretation of stereophotos. Subtle and gradual changes
in environmental conditions are often difficult to discern via stereoscoping,
and it is easy to misplace the boundaries of soil polygons in the manual
delineation process. Thus, the mapping process is not only tedious and
time consuming, but can also be error-prone and inconsistent. In addition,
the entire soil map production process must be repeated for each future
soil survey update.

Lack of documentation

A third limitation is the lack of documentation on soil-landscape models
for areas mapped. The key concerns here are the extent to which the soil-landscape
model is being documented and the extent to which experience (knowledge)
for a given area is being passed from one generation of soil mappers to
the next. In most cases, knowledge of the soil-landscape model of an area
is lost when the soil mapper retires or moves out of the area. The new
surveyor must either start over from scratch, or must try to uncover original
concepts only implicitly represented in the survey.

Summary

As a result of the above constraints, the traditional method has limited
field soil scientists' ability to express their understanding of soil
variation over space in an efficient and speedy way.